Ocean thermal energy conversion (“OTEC”) is a method for generating electricity based on the temperature difference that exists between deep and shallow waters of a large body of water, such as an ocean, sea, gulf, or large, deep lake. An OTEC system utilizes a heat engine (i.e., a thermodynamic device or system that generates electricity based on a temperature differential) that is thermally coupled between relatively warmer shallow and relatively colder deep water.
One heat engine suitable for OTEC is based on the Rankine cycle, which uses a low-pressure turbine. A closed-loop conduit containing a fluid characterized by a low boiling point, such as ammonia, is thermally coupled with warm water at a first heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor is forced through the turbine, which drives a turbo-generator. After exiting the turbine, the vaporized working fluid is condensed back into a liquid state at a second heat exchanger where the closed-loop conduit is thermally coupled with cold water. The condensed working fluid is then recycled through the system.
OTEC systems have been shown to be technically viable, but the high capital cost of these systems has thwarted commercialization. The heat exchangers are the second largest contributor to OTEC plant capital cost (the largest is the cost of the offshore moored vessel or platform). The optimization of the enormous heat exchangers that are required for an OTEC plant is therefore of great importance and can have a major impact on the economic viability of OTEC technology.
Shell-and-tube heat exchangers have been considered for use in OTEC applications because of their potential for large volume fluid flow and low back pressure. Common industrial shell-and-tube heat exchangers consist of multiple tubes placed between two tube plates and encapsulated in a pressure vessel shell. Fluids or gasses of differing temperatures are passed through the heat exchanger, transferring the heat energy from one medium to the other. The tubes may be press fit or welded into the tube plates.
In many applications (e.g., OTEC, nuclear power generation systems, and chemical plants), separation of the liquids or gasses is critical. As a result, the tubes must be welded in place, typically using fusion welding, and checked for leaks prior to entering service.
The process of welding the heat exchanger tubes has many drawbacks including: (1) high labor costs to prepare and weld all of the tube joints via manual fusion welding techniques, (2) defect repair costs due to the complex manual or semi-automatic fusion welding process, and (3) increased corrosion susceptibility due to dissimilar materials included in the fusion-weld. For OTEC systems, in particular, galvanic-corrosion is a major cause of working fluid contamination due to the exposure of the welds to seawater.
Through the mid 80's to the early 90s, aluminum and aluminum alloys were tested in an actual OTEC environment to determine their compatibility with an ocean environment. These instrumented and remotely monitored tests correlated heat-transfer performance and seawater chemical and physical properties with corrosion in the heat exchangers. As a result of this extended testing, it was concluded that several relatively inexpensive aluminum alloys should survive well in an OTEC application.
The form factor for the heat exchangers being tested was mostly shell-and-tube type. Unfortunately, it was concluded that fabricating shell-and-tube heat exchangers of sufficient surface area out of aluminum would be prohibitively expensive. “Roll bond” heat exchanger panels were proposed as an alternative, which provide the larger surface areas required for OTEC applications at roughly twenty percent the cost of equivalent shell-and-tube units. Corrosion and biofouling issues, however, continue to plague heat exchanges based on roll-bond panels.
In 1989, roll-bond panels were inserted into some the heat exchangers that were being tested in the OTEC environment. This testing led to the development of roll-bond type heat exchanger panels that were actually installed in a 50 kW plant built in 1996. During the first year of testing, serious ammonia leaks were experienced due to corrosion. The corrosion was due to electrolysis, which was caused by the spacer material between the aluminum panels.
By the mid-1990s, government funding of OTEC had concluded. Remaining hurdles for compact aluminum heat exchangers at that time included concerns over the placement of brazed sections within a heat exchanger core.
With today's growing need for energy, using a renewable constant source is a desirable solution. Currently, OTEC power plants having power generation capability of up to 100 MW are being proposed. Such systems, however, will require a very high volume of seawater flow. As a result, many large, high-efficiency heat exchangers will be required and there is a renewed interest in shell-and-tube heat exchangers suitable for use in seawater environments. Unfortunately, development of a shell-and-tube heat exchanger, suitable for OTEC applications, that accommodates high flow rates while minimizing pumping parasitic losses and offering long life in the ocean environment remains elusive.